available from measurements a t 280 nip, but were not sufficiently accurate and were not used. The curvature of the log c, us. t plot was about the same for 8.8 and 9.2% of slow component; it was greater at 17.3% and greatest at 65%. The curve for 8.8% slow component was used for determining A log ci in intervals of 20 minutes. The plot of these values of A log c1 us. time is shown in Figure 2, h; the curvature is of the kind expected for a more slowly dialyzing impurity (Figure 2, g, lower curve). Discussion. The sensitivity of t h e dialysis technique in t h e detection of heterogeneity is greater for more slowly than for more rapidly dialyzing impurities. Because t h e more sloivly dialyzing impurities accumulate in proportion as dialysis proceeds, t h e sensitivity can be increased without limit. For example, t h e log c1 us. t relationship obtained with t h e residual material after the bulk of solute has been removed from a concentrated solution by dialysis or by filtration through a series of membranes (3-6) might be compared to the log c, us. t relationship obtained with the original solution after appropriate dilution. In a single experiment, however, the sensitivity of the dialysis technique appears to be less than that of the heterogeneity
tests that can be made with free boundary diffusion (1, 7 ) . The ease of the technique, the lack of restrictions in the choice ‘of the method of chemical analysis, and the low cost of the equipment are advantages over the freeboundary diffusion method. If a specific quantitative test is available, dialysis rate constants may give information on molecular size without the necessity of prior isolation of the substance in question. While there is a simple relationship between the third root of the molecular weight, or molecular volume, and the diffusion coefficient (10, 11), such a relationship should not be expected for dialysis rate coefficients, since according to Craig (3-6) the membrane exerts a selective action which enhances the difference in rate found by diffusion alone. For this reason and because of the variability of membranes, substances of known molecular weight must be used for reference. Binding to nondialyzable material may be avoided by using uItrafiltrate.
preparing the photographs of the figures. The aid of Raymond S. Ruffin in the design of the apparatus is gratefully acknowledged, LITERATURE CITED
(1) Akeley, D. F., Gosting, L. J., J . Am. Chem. Soc. 75, 5685 (1953).
(2) Canals, E., Marignan, R., Bardrt,
(3) (4)
(5) (6)
L., Ann. pharm. jranc. 13, 645 (1955). Craig, L. C., King, T. P., J . A 4 t ~ l . Chem. SOC.77, 6620 (1955). Zbid., 78, 4171 (1956). Craig, L. C., King, T. P., Stracher, A , , Federation Proc. 16, i2-I (!957). Craig, L. C., King, T. P., Stracher, A , J . -4tn. Chem. SOC.79, 3720 (19573. Hoch. H.. S r c h . Biochem. and Bio phys. 53, 387 (1954). Huckel, Raker, “Structural Cheni istry of Inorganic Compound..” Elsevier, New York, 1950 Kortum, G., Bockris, J. 0’11 , “Textbook of Electrochemistrv,” pp. 667, 680, Elsevier, KeT Toil,, 1951. Longsworth, L. G., J . L4m. Chei . SOC.74, 4155 (1952). Zbzd., 75, 5705 (1953).
ACKNOWLEDGMENT
The authors wish to thank William T. Ham, Jr., for constmctive criticism, and Sidney Solomon for valuable discussion, and Frederick H. Schmidt for
RECEIVEDfor review June 19, 1957. Accepted March 10, 1958. n70rk made possible by an A. D. Williams fellowship and research grant, LIedical College of Virginia.
Permselective Membrane Electrodes 0
Ana lytica I Applications JAMES S. PARSONS Research Division, American Cyanamid Co., Bound Brook, N. 1.
b Cation- and anion-selective membranes were made by molding commercial ion exchange resins with a plastic binder. The sulfonated-type resins provide cation permeability; quaternarized amine resins, anion permeability. Electrodes made of these membranes show almost theoretical response over a limited concentration range to ions of a single species without interference of ions of opposite sign, but little selectivity to ions like sodium and potassium A or other cations in a mixture. sulfonated polystyrene membrane was studied as a p N a electrode. Electrodes were investigated as indicator electrodes for the potentiometric titration of sulfate with barium acetate. Good end points were obtained only in the absence of many other cations and anions. 1262
ANALYTICAL CHEMISTRY
P
ERMSELECTIVE
MERIBRAVE
&C-
trodes have attracted considerable attention in the biological (10) and colloid fields for measuring activity of various cations and anions. Marshall (6, 7 ) employed clay membranes to determine cations. Sollner ( I S ) , Gregor ( 5 ) , and Carr (1) recommend collodion-base membranes impregnated with polyelectrolytes for both cation and anion measurements. Xembranes made of ion exchange resins incorporated in a plastic binder were introduced by Wyllie and Patnode (15, 16), Schindewolf and Bonhoeffer (Q), and Sinha (11 ) . Membrane electrodes, if suitable for determination of individual cations and anions in the sense of the glass electrode for p H measurements, would provide useful analytical tools. Actually good membrane electrodes (16, 16) can be
made n-hich show almost theoreticnl Sernst equation response to ions of ‘1 single species without interference of ions of opposite sign, but little selectivity to ions like sodium and potassium (11) or other cations in a mixture. This paper presentq analytical studies on membrane electrodes and treats some of the approaches aimed toward making the electrodes more selective. One promising application was the use of a sulfonated polystyrene membrane in the sodium form as a pXa electrode. Another deals with membranes as indicator electrodes in the potentionietric titration of sulfate with barium ions. Recently, Sinha ( 1 1 ) showed that an anionpermeable membrane can serve as a n indicator electrode for sulfate titration. The present paper treats both cationand anion-permeable membrane indicator electrodes for sulfate titrations.
*
Membranes for construction of electrodes were made along the general lines recommended b y Wyllie and Patnode (15, 16)-i.e., finely ground ion exchange resins were molded with a plastic binder. Wyllie and Patnode made their membranes for use as a sodium ion electrode; their electrodes gave the highest concentration potentials a t higher concentration of sodium chloride than other membranes described in the literature. Xeihof ( 8 ) indicates t h a t the plastic bonded ion exchange membranes are superior to other types because they swell less and thus have higher selectivities than ion exchange resin membranes, which can swell with no restraint except their own cross linking. The theory and origin of the membrane potential are treated in several reviews (10, 13). Marshall (6, 7) gives cyuations for the treatment of the membrane potential of mixtures of ions. He indicates that mixtures can be determined by using different membranes n here, for instance, the ratio of the mobility of the sodium ion to the potassium ion is differwt for two membranes. By making two potential nirasurements with the two membrane electrodes, simultaneous equations may be solved to give the activity of each cation. dctually, attempts t o make membranes (11, 15, 16) with large enough difference in sodium-potassium mobility ratio to give a quantitative separation h a r e not been successful and this is still an active research area. One approach has been to reduce pore size so as to exclude larger ions by an ion sieve effect. For a p S a electrode, this factor cannot br usrd, because the hydrated sodium ion is larger than the hydrated potassium ion. Another approach has been t o reduct. the conductivity or transference of the interfering ion in the membrane by introducing a suitahle complexing group in the iqmbrane structure. Carr and Engelstad ( I ) have shown that collodion rnenibrnnes impregnated with sodium polyphoqphate are selectix e to alkali cations in the presence of alkaline earths; hon ever, they are not selective to individual alkali cations like sodium and potassium. Carr and Engelstad point out that selectivity is due to a relative increase in the transference of alkali cations compared with calcium and magiiclsium, a situ:ition brought about by the complexing action of polyphosp ha t e , Koermann, Bonhoeffer, and Helfferieh (14) have prepared a membrane electrode from a potassium-selective ion exchange resin. This resin contains the dipicrylamine grouping in the polystyrene molecule and was originally synthesized by Skogseid (12). The resin as a nienibrane electrode showed
8ECKMAN C A L O M E L ELECTRODE (FIBER TYPE)
R , EF.
SOL'N.
APIEZON W A X
\MEMBRANE
Figure 1.
Electrode
used to give a disk about 1.9 cm. in diameter and 1 mm. thick. Membrane disks were made Eith Amberlite IR-120, Dowex 50 (colloidal), Amberlite IRA-400, and Dowex 1. These resins, when molded with an equal part of polystyrene, gave a mechanically satisfactory membrane. The commercial membranes Amherplex C-1 and Amberplex A-1 (Rohm & Haas Co.) were obtained in sheets (0.6 mm. thick). Electrodes were constructed by sealing the membrane disks (after soaking in the appropriate electrolyte solutions) on the flanged end of a test tube 1.8 cm. in diameter with hot Apiezon wax (K-100, James G. Biddle Co., 1316 Arch St., Philadelphia). The closed end of the test tube was cut off a t a convenient length for inserting a Beckman (fiber type) calomel electrode as shown in Figure 1. APPARATUS
poor selectivity toward potassium ion in presence of sodium ion. An attempt was made to incorporate the tetraphenylboron ion along with the sulfonated resin in a membrane, to reduce the mobility of the potassium ions based on the insolubility of the potassium salt. So far the results have not been promising. One of the problems is finding a good means of supporting or fixing chemically or physically the tetraphenylboron ion in the membrane structure. Gregor and Schonhorn (4) indicate that membranes made of multilayers of barium stearate shov specific response to barium ions in presence of sodium ions. PREPARATION OF MOLDED MEMBRANES
T h r dried ion exchange resins and granular polystyrene were ground by repeated passing through a hIikropulverizer until an appreciable amount of the material could be passed through a 325-mesh screen. D r y ice was used to prevent the mill from heating and clogging. The dried ion exchange resins (vacuum at 60" C.) and nolvstvrene rvere screened through 325' nlesK and mixed thoroughly before molding. The mixture was placed in the mold, and the prpssure was adjusted to 1000 p.s.i.g. K h e n the temperature reached 120" t o 130" C., the pressure n-as increased t o 6000 p.s.i.g. and the temperature was increased to 185" C. As soon as the temperature reached 185" C., the heating element was sn-itched off and the mold was allowed to cool to 50" to 60" C. before the pressure was released. The disk was removed by tapping lightly n i t h a wooden stick. The disk often fractured on removing, if the mold was allowed t o cool t o room temperature. The disks were exposed to water vapor before soaking in solutions, in order to prevent them from shattering. About 0.4 gram of a 50-50 mixture of ion exchange resin and pol! stj-rene was
The molding press consisted of a cylindrical steel block, 21/2 inches in diameter and 2 inches tall, with a center bore to accommodate a plunger 0.75 inch in diameter. The cylinder had mire wrapping for electrical heating. A thermocouple was located in a small bore about 3,'16 inch from the center bore wall. The mold vas placed in a Carver hydraulic preqs for obtaining the indicated gage pressures (pounds per square inch). Asbestos board insulation was used a t the top and bottom of the mold, to reduce heat conduction to the press. Potential measurements were made with a high impedance type direct reading p H meter (Leeds & Northrup Catalog S o . T664). With this meter a full scale reading of 70 mv. vias obtained by placing a 50-ohm resistor in the automatic temperature compensator outlet. The scale was calibrated from the output of a Ruhicon potentiometer (Serial Y o . 32355). Potentials were ebtiniated t o k 0.2mv. A magnetic stirrer was employed in the outside solution for all potential measurements. ;\latched Beckman calomel electrodes (fiber type), which have a slow potassium chloride leak, TI ere used on opposite sides of the membrane. DIRECT DETERMINATION
OF
SODIUM IONS
Preliminary studies were made on a cation-permeable ion exchange membrane for determination of sodium ions in the presence of other ions. This membrane &as a 50-50 mixture of Amberlite IR-120 and polystyrene, molded according to the procedure described in the section on preparation of membranes. It was converted to the sodium form by soaking in sodium chloride colution. A plot of potential measurements (25" C.) for various sodium chloride concentrations expressed as p y a is shown in Figure 2. The reference solution was 0.01M sodium chloride. The response is linear and conforms closely to the Nernst equation for a range of VOL. 30, NO. 7,JULY 1958
1263
I
I
I
10-3
10-4
MOLARITY
Figure 2.
2.0 2.0 2 0 a
pSa Apparent 1.6
~~
100 100 25 7 5
Effect of Sodium and Potassium Chloride
Salt hdded, Sac1 SaCl SaCl Sac1 KC1
co
Error
Ng.
i5 150
-2.2 -2 9 -3 8
300 372
-7.6
i~
-4
Literature
- 2 . 8 (KaC101) -6 3 (KCI)
about 3 to 4 powers of 10. Deviations from linearity occur in the vicinity of l J 4 and above, or below l O - 4 M . The circles are experimental points. The triangles were calculated according to E = RT/nF In aJa2
The squares are experimental values using the same membrane after 6 months' storage in dilute sodium chloride. The deviations above 1-11 are due to the inefficiency of the membrane -that is, chloride gets into the membrane as sodium chloride. The reference solution (0.01M sodium chloride) in the membrane tube (Figure 1) must be changed frequently and potential measurements made fairly rapidly (1 minute) in order to obtain reproducible results. A small amount of potassium chloride which diffuses from the fibertype Beckman calomel electrode contributes to the potential. This effect is greater for the solution in the membrane tube, because of the smaller volume. On the other hand, if 0.1M sodium chloride is used in the membrane
1264
e
Figure 3.
ANALYTICAL CHEMISTRY
20
\
Effect of sodium and potassium chloride
Effect of Cations
16 10
IS
mi. O.IM BoCI,
Dowex 1 (hydroxide form).
Table II.
10
No CI
Add, Mg. HqSO, HiSO; >fgSO* Ca(XOa)n (SH,),SOd KC1
1
IO
p N a electrode
Table I.
pSa 2.0 2.96 2.02
I
10-1
10-2
NO NaCl
A
195 1.65
1.55 1.73
Add ROHa ROH ~. . Versene ROH Versene ROH ROH heat Barium tetra henvlboron, &rseLe, ROH
++ +
pSa 2.02 3. 00 ~~
2.00
2.00 1.98
2 0
tube with 0.OlM sodium chloride (about 50 ml.) outside, the potential does not change significantly in 15 minutes. The 0.1M concentration is less desirable as a reference solution, as the potential range would be much greater for the lower concentrations and there would be more serious deviations from theoretical Nernst equation. Long-range reproducibility data over the complete concentration range shown in Figure 2 have not been obtained. Table I shows the effect of the p S a of various added cations, by the apparent pNa values obtained in column 3. The volume of solutions 11-as 50 to 70 nil. The last two columns show how this interference can be prevented. The solution is returned to the original pNa value by addition of a strong base anion exchange resin in the hydroxide form to remove hydrogen ion or Versene (acid form) along with the resin to remove calcium and magnesium. Potassium ion interferes but can be removed by precipitation with barium tetraphenylboron. POTENTIOMETRIC TITRATIONS OF SULFATE
When permselective membranes are used as indicator electrodes in titrations, good selectivity to individual ions is not necessarily a requirement, so long as empirical changes are obtained in the vicinity of the end point. The titration of sulfate ion with barium ions was studied using both cation-permeable and anion-permeable membrane electrodes.
C ation-Perm ea bl e Membrane. Commercially available Amberplex C-1 membranes were used for construction of indicator electrodes. Xolded cation-permeable membranes ( h i b e r l i t e IR-120) were studied; e w e p t for their superior mechanical characteristics they offered no advantages over the $mberplex C-1 membranes. The membrane electrode n-as stored in 0.lM potassium chloride, m-hich was used as the reference solution. For satisfactory electrode response the electrode vias soaked in 0.01M barium chloride for 10 minutes and then about 1 minute in 0.05;M magnesium sulfate before each titration. The electrode assembly was similar to that shown in Figure 1. Another saturated calomel electrode was immersed in the sample solution. Curve A , Figure 3, was obtained by titration of 5 meq. of sulfuric acid in 50 to i o ml. of n-ater with 0.1M barium chloride, using a cation-permeable membrane-Le., the sulfonated polystyrene type. Prior to the titration, sulfuric acid was neutralized to p H 4 (methyl orange) with magnesium acetate.
Interpretation of the potential during the course of a titration in terms of cations responding a t the membrane surface is difficult. Potassium ions are on one side of the membrane, while magnesium ions and barium ions are on the other side. This would represent a multiionic potential involving both monovalent and divalent cations, for which equations treating such systems have not been satisfactory. Five titrations lvith sample sizes 4 to 3.6 meq. gave a percentage recovery ranging 99.0 to 100.1 (average 99.5) with a standard deviation of +0.4%. The end point was taken a t the volume giving the maximum rate of potential change. At the end point the potential change was about 4 mv. per 0.1 ml. of 0.1-Jf barium chloride. Better reprcducibility was obtained by adding most of the barium reagent before immersing the electrode; otherwise a blank correction was necessary. The effect of sodium ion on the titration is shown by curves B , C, D, and E
in Figure 3 and potassium ion by curve F . Besides changing the shape of the curve, sodium and potassium cause large coprecipitation errors, as shown in Table 11. Fritz and Freeland (3) reported (see column 3) coprecipitation errors of about the same order of magnitude using adsorption indicators.
Table 111.
Effect of Various Substances with Anion Membrane Electrode
Added, R l l . 0 3 3 5 SaOH 5 XaC1 100 mg. 0 1X HgAcn 15 1 S NHCl 2 0 1JP HgAcz 20 Sone
An anion-permeable membrane in t h e sulfate form was also studied as an indicator electrode in t h e titration of sulfate with barium acetate. T h e membrane mas made b y molding a Anion-Permeable Membrane.
0.1.h- HlSOd, Afl. Taken Found 50 00 48 64 30 00
28 84
96 1
30 00
29 34 29 98 40.12
97 8 99 9 100.3
30 00
40.00
Sone
%
Recovery 97 3
fcre with the visual adsorption indicator methods. Eisenman, Rudin, and Casby ( 2 ) have recently described a sodium aluminosilicate glass electrode which shows remarkable selectivity for sodium ion relative to potassium ion. ACKNOWLEDGMENT
3021
22
23
24
ml. 0.1 M. BARIUM ACETATE
Figure 4. Titration of sulfate with barium acetate, anion-permeable membrane
50-50 mixture of Amberlite IRA-400, a strong base anion exchange resin of t h e quaternary ammonium type, with polystyrene. T h e membrane n-as converted to the sulfate form and the electrode constructed thereof n-as kept soaking in 0 , l M potassium sulfate. The latter solution was used as the reference solution. Figure 4 shows the peak-type break obtained in the vicinity of the end point. Five milliequivalents of sulfuric acid in 50 ml. n i t h 1 ml. of glacial acetic acid present was titrated with 0.11ilf barium acetate. The peak in Figure 4 was taken as the end point, Five titrations with sample sizes of 4- to 5meq. quantities of sulfuric acid gave percentage recoveries ranging 99.8 to 100.7 (average 100.1), with a standard deviation of =k0,4%. The titration must be carried out relatively slowly in order to obtain the peak-type break. B few experiments were carried out in which titration curves were recorded a t a f l o ~rate of 0.84 ml. per minute, using a constant feed syringe-drive mechanism. The breaks were somewhat poorer than for the manual titration. The effect of various other substances is presented in Table 111. Hydrochloric and nitric acids level the peak-type end point break. Hydrochloric can be removed with mercuric acetate. However, a poor end point break and lower recovery was obtained with mercuric acetate present. Sodium
and potassium ions gave the usual large coprecipitation errors. $nother anion-permeable membrane made of Dowex 1 gave a titration curve with end points corresponding more exactly with the ascending curve than the peak as shown in Figure 4. dpparently, the response is somewhat different from that of the IRA-400 membrane. Four titrations of 2.5- to 5.0-meq. quantities of sulfuric acid gave a recovery of 99.5% with a standard deviation of lt0.370. SUMMARY
Membrane electrodes made of commercial ion exchange resins can be useful for concentration measurements in simple electrolyte systems or in mixtures, if care is taken to remove interfering ions. They show little selectivity to individual ions like sodium and potassium or other individual cations in a mixture. Progress in the synthesis of more selective ion exchange resins and a better understanding of membranes may ultimately provide selective membranes electrodes. Their successful application as indicator electrodes in the titration of sulfate with barium ions depends on the absence of many other cations and anions. The adsorption indicator methods suffer similar disadvantages. However, the membrane electrode may have a n advantage where color is present in simple solutions, which would inter-
The author m-ishes to thank his colleagues in the American Cyanamid Co. for their many helpful suggestions and especially to thank Charles Alaresh and Killiam Seaman for the interest and encouragement given this study. He nishes to thank H. C. Lawrence for constructing the membrane mold. LITERATURE CITED
W.,Engelstad, IT., 30th National Colloid Sympogiuni, llivision of Colloid Chemistry, ACS, University of Wisconsin, June 18-20, 1956. (2) Eisenman, G., Rudin, D. O., Casby, J. U., Sczence 126, 831-4 (1) Cam, C.
( 19.57) , \ - - -
(3) Fritz, J. S., Freeland, RI. Q., AXAL.
CHEU.26, 1593 (1954). H. P., Schonhorn, H., J . Am. Chem. SOC. 79, 1507 (1957). Gregor, H. P., Sollner, K., J . Phys. Chem. 58,409 (1954). Marshall, C. E , I b i d . , 48, 67 (1944). Xlarshall, C. E., Bergman, IT. E., Zbzd., 46, 52 (1942). Seihof, R., Zbid., 58, 916 (1954). Schindewolf, U., Bonhoeffer, K. F., Z . Elektrochem. 57, 216-21 (1053j. Shedlovsk\-, T., “Electrochemistry in Biology” and Medicine,” Kiley, Xeiv York, 1955. Sinha, S. K., J . I n d i a n Chem. Soc. 32. 35-8 (1955). , , Skogseid,- A , , dissertation, Oslo, Sorway, 1948. Sollner, K., J . Electrochem. SOC.97, 139C-51C (1950); Ann. S . Y . Scad. Sci. 57, 177-203 (1953). Koermann, D., Bonhoeffer, K. F., Helfferich, F., 2. phys. Chem. 8 , 265-83 (1956); Discussions Faraday SOC.No. 21, 217 (1956). Wyllie, M. R. J., J . P h y s . Cheni. 58, 67-80 (1954). Wyllie, 11. R. J., Patnode, H. W., I b i d . , 54, 204 (1950). RECEIVEDfor review October 1, 1957. Accepted ?\larch 11, 1958. Division of Analytical Chemistry, Fisher Award SgmposiumHonoring JohnH. Yoe, 131st Meeting, ACS, Miami, Fla., April 1957. (4) Gregor,
VOL. 30, NO. 7, JULY 1958
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